The present invention relates generally to tracking systems that use magnetic fields such as for surgical interventions and other medical procedures. More particularly, the present invention relates to techniques for the correction of multiple electromagnetic sensor positions.
Tracking systems have been used to provide an operator (e.g., a physician) with information to assist in the precise and rapid positioning of a medical (e.g., surgical) device in a patient's body. In general, an image is displayed for the operator that includes a visualization of the patient's anatomy with an icon or image representing the device location projected thereon. As the device is positioned with respect to the patient's body, the displayed image is updated to reflect the correct device location. The image of the patient's anatomy may be generated either prior to or during the medical or surgical procedure. Moreover, any suitable medical imaging technique, such as X-ray, computed tomography (CT), magnetic resonance imaging (MRI), positron emission tomography (PET), ultrasound, endoscopy, and optical imaging in the UV, visible, and infrared light spectrums may be utilized.
When used with existing image sets, these previously recorded diagnostic image sets themselves define a three dimensional rectilinear coordinate system, by virtue of their precision scan formation or the spatial mathematics of their reconstruction algorithms. Accordingly, to accurately depict the device position and orientation with the external coordinates of the device being employed, the coordinates of the image should be correlated with the external coordinates of the device being employed.
So that transformations between the respective coordinates may be performed, the various sets of coordinates may be defined by robotic mechanical links and encoders, or more usually, are defined by a fixed patient support, two or more receivers, such as cameras which may be fixed to the support, and a plurality of signaling elements (e.g., electromagnetic sensors) attached to a guide or frame on the device that enable the position and orientation of the device with respect to the patient support and camera frame to be automatically determined by triangulation. Three-dimensional tracking systems employing two video cameras and a plurality of emitters or other position signaling elements have long been commercially available and are readily adapted to such operating room systems. When tracked markers appear in the diagnostic images, it is possible to define a transformation between operating room coordinates and the coordinates of the image.
In one typical tracking system, an electromagnetic (“EM”) transmitter is fixed in relation to a patient's body, an EM receiver is fixed in relation to the device, and another EM receiver is fixed in relation to the C-arm of an X-ray fluoroscopy system. The EM transmitter generates an electromagnetic field that is detected by the EM receivers. The signal received by the receiver fixed in relation to the imaging system may be suitably processed to determine the receiver's position and orientation. As this receiver is fixed in relation to the imaging system, the receiver may then be used to determinate the position and orientation of the C-arm. Accordingly, once the signal received by the receiver fixed in relation to the device is processed, the device receiver's position and orientation may be correlated to the imaging system so that the device's position and orientation may be projected onto the diagnostic image.
As will be appreciated, correlation of the patient anatomy with the diagnostic image may be complicated by a number of factors. For example, the presence of field distorting objects (e.g., a C-arm, X-ray detector, or surgical table) may result in distortions in the magnetic field emitted from the EM transmitter and thereby change the magnitude and direction of this field. For example, the presence of a signal from another source, the magnetic field of the eddy current in a conductive object, or the field distorting effect of a ferro-magnetic object can result in these distortions. Unless compensated for, these distortions will result in error in the measured position and orientation of the receivers. While distortion maps are generally used to compensate for certain distortion objects, all the distortions may not be fully compensated for using existing techniques. In other words, there may be some residual distortion that is not compensated for by the distortion map. In addition, as the distance between the transmitter and receiver increases, the signal-to-noise ratio of the sensed data worsens. For example, a transmitter-to-receiver distance of greater than about eighteen inches (approx. 500 mm) may yield unreliable data for certain implementations with existing technologies. As such, if the transmitter-to-receiver distance increases, for example, due to movement of the C-arm, a reliable position and orientation for the C-arm may not be obtained. Without a reliable position and orientation for the C-arm, the device position and orientation may not be accurately projected onto the diagnostic image.
Accordingly, a tracking system utilizing multiple EM receivers fixed in relation to the imaging system may be used to account for some of these complicating factors. For example, the uncompensated distortions may not impact all of the EM receivers in the same manner so that one or more of the receivers may return acceptable data even where one of the receivers does not. Alternatively, dependent upon the position of the C-arm, one of the EM receivers may have an acceptable transmitter-to-receiver distance even if the other receiver does not. However, conventional techniques are generally designed for tracking systems that include a single EM receiver fixed in relation to the C-arm.
Accordingly, there is a need for an improved technique for the correction of magnetic field distortions. Particularly, there is a need for a technique that corrects for magnetic field distortions in tracking systems that utilize multiple EM receivers.